FAST P-WAVE PROPAGATION IN SUBDUCTED PACIFIC LITHOSPHERE - REFRACTION FROM THE PLATE

P waves traveling from events in the Tonga-Kermadec seismic zone to stations in New Zealand are very fast with highly emergent, dispersed waveforms. Ray tracing has shown the waves to travel close to the subducted Pacific plate throughout their length, and synthetic seismogram calculations have shown the dispersion requires a very thin (8-12 km) fast layer. Previous work has been based on data from analogue records and one digital, single-component short-period instrument; no polarization analysis was possible, and measurements of dispersion were Limited by the bandwidth. From January 1991 to August 1992 we deployed nine broad band, three-component seismometers in good sites for observing these arrivals; the data are augmented by three-component, short-period digital records from new stations of the New Zealand National Network. In this study we analyze 1191 broad-band and 2076 short period Seismograms from 71 events for polarization of the initial P wave. The polarization directions are found to be up to 300 off the great circle path and consistently steep (200 from vertical). They are too steep to be explained by standard ray paths or refraction from a fast horizontal layer. We invert the polarization directions for a tilted interface beneath the array and use arrival times to control the depth to the interface, which is found to lie close to the top pf the subducted plate inferred from the seismicity. Wet conclude that these precursive, emergent P waves have traveled through a fast layer close to the top of the subducted plate and refract upward to the station. A second arrival, with lower dominant frequency near 1 Hz and normal travel time, is occasionally seen on both broad band and Short-period stations, Its polarization direction is similarly steep but difficult to measure; the evidence suggests that it also travels within the plate with similar ray path to the precursor.